When a compression test yields Cylinder 3 showing 80 psi while others are at 150 psi, what is the likely cause of compression loss?

When a compression test shows one cylinder with significantly lower pressure compared to others, it typically indicates a problem within that specific cylinder. In this case, Cylinder 3 reads 80 psi, while the others are at 150 psi. This substantial difference suggests that there is a loss of compression, which can stem from a few potential issues related to the engine's internal workings.

The most likely cause here is faulty valve seating. If the intake or exhaust valves in Cylinder 3 are not seating properly, it can lead to leaks during the compression stroke. This kind of leakage prevents the cylinder from achieving the same pressure as the other cylinders, which is reflected in the low 80 psi reading. Proper valve seating is crucial for maintaining the necessary seal needed for optimal compression, and any issue here directly affects cylinder performance.

While a blown head gasket or leaking intake valve could also lead to compression loss, they often present with additional symptoms, such as coolant loss or visible leaks, which are not specified in this scenario. A faulty spark plug would usually affect firing rather than compression directly, and typically wouldn't result in such a stark difference in cylinder pressure. So, the most straightforward explanation for the low compression in Cylinder 3 is that it is related to faulty valve seating.